The present invention relates generally to magnetic devices in microelectronic circuits, and, more particularly, to magnetic field concentrators for magnetic random access memory devices and method of forming the same.
Magnetic random access memory (MRAM) is a non-volatile memory technology that offers substantial benefits in many applications over traditional memories such as dynamic random access memory (DRAM) and flash memory. The speed of MRAM devices, combined with the non-volatile nature of its storage will eventually allow for “instant on” systems that come to life as soon as the system is turned on. This saves the time and electrical power consumed in transferring boot data from a slow non-volatile memory into faster memory capable of supporting a microprocessor.
The magnetic memory element in the most promising MRAM devices is realized as a magnetic tunnel junction (MTJ). The MTJ is a structure having ferromagnetic layers separated by a thin insulating tunnel barrier. Digital information is stored and represented in the memory element as directions of magnetization vectors in the magnetic layers. More specifically, the magnetic moment of one magnetic layer (the “reference” layer) is fixed or pinned, while the magnetic moment of the other magnetic layer (the “free” layer) may be switched between the parallel direction and the antiparallel direction with respect to the fixed magnetization direction of the reference layer. Depending on the state of the free layer (parallel or antiparallel), the magnetic memory element exhibits two different resistance values: low resistance for parallel, and high resistance for antiparallel. Accordingly, detection of the value of resistance allows an MRAM device to provide information stored in the memory element.
The free layer magnetization direction is adjusted to be parallel or antiparallel through the use of a magnetic field strong enough to reorient it without being strong enough to affect the orientation of the reference layer magnetization. In conventional MRAM devices, this so-called “write” field is generated by driving current through on-chip conductive wires (“write” wires). Typically, generating fields sufficient to switch the free layer requires undesirably large amounts of current, as the on-chip wires may be a significant distance from the magnetic layers, and a large portion of the generated flux is lost to regions away from the magnetic layers.
One improvement to this situation is through the use of additional magnetic films surrounding portions of the write wires to provide a low reluctance path for focussing the magnetic flux on the MTJ free layer. A compromise between manufacturability and flux focussing efficiency typically dictates that the low reluctance path be made as a horseshoe-shaped (in cross-section) liner of high permeability magnetic material surrounding the write wire on all sides but the side facing the MTJ. (U.S. Pat. No. 6,559,511, N. Rizzo, “Narrow Gap Cladding Field Enhancement for Low Power Programming of a MRAM Device”) The magnetic flux surrounding the write wire can thus be focussed to escape primarily in the desired direction—towards the free layer.
There exist significant manufacturing complexities in physically realizing such magnetic liners in microelectronic circuitry, particularly for wires residing above the MTJ films (where the horseshoe opening points down). The most common method suitable for scaling to small dimensions (deep submicron) involves the following processing steps: 1) etch a trench in the dielectric film which will encase the write wire, 2) deposit magnetic liner material to coat the sidewalls, and undesirably, the bottom of the trench, 3) anisotropically etch the liner material to remove preferentially the liner material at the bottom of the trench, while leaving the sidewall liner largely intact, 4) deposit the write wire conductor (e.g., copper) and pattern with e.g., a Damascene technique, and 5) cap the wire with electroless-plated magnetic liner material. The most difficult process in this sequence is step number 3, anisotropically etching to remove the liner material at the bottom of the trench without substantially modifying the material on the sidewalls.
Methods of patterning or modifying certain magnetic films in another application (not in a “magnetic liner” application) without explicit etching have been demonstrated recently. These include ion implantation into the magnetic materials “Track Width Definition of Giant Magnetoresistive Sensors by Ion Irradiation,” by Liesl Folks et al., IEEE Transactions on Magnetics, vol 37, No. 4, July 2001, pp 1730–1732, “Localized Magnetic Modification of Permalloy Using Cr+ Ion Implantation,” by Liesl Folks et al., J. Phys. D: Appl. Phys., vol 36, November 2003, pp. 2601–2604 and oxidation of the magnetic materials “Magnetic Tunnel Junction Pattern Technique,” by Eugene Chen et al., J. Appl. Phys., vol 93, No. 10, May 2003, pp. 8379–8381. These methods have not been used to date for patterning films in magnetic liner applications.
The industry would benefit greatly from an improved method of removing the magnetic liner material at the bottom of trenches, so as to be able to more efficiently create magnetic liner shapes that focus the write fields onto the free layer of MTJs.